Optical substrate, light emitting element, display device and manufacturing methods thereof

ABSTRACT

The present invention relates to an optical substrate comprising a transparent substrate, a low refractive index layer, whose refractive index is lower than that of the transparent substrate, disposed over the transparent substrate, and a solgel film disposed over the low refractive index layer; a light emitting element having a first electrode, a light emitting layer and a second electrode over the solgel film of this optical substrate; and a display device provided with this light emitting element.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical substrate for light emittingelements, a light emitting element using it, a display device using thelight emitting element, and manufacturing methods thereof.

2. Description of the Related Art

Along with the advance of information and communication technology overthe recent years, diverse display devices have been developed. Amongthem is the self-luminescent organic electroluminescence (EL) element,which is attracting interest for its high display quality and thinness.

The organic EL element is a self-luminescent element embodying theprinciple that recombination energy between holes injected from theanode and electrons injected from the cathode causes fluorescentsubstance to emit light by applying an electric fields. Since a reportwas published on a laminated low voltage-driven organic EL element, manyresearch attempts have been made on organic EL elements composed of oneorganic material or another. One example is an element usingtris(8-quinolinol) aluminum for the light emitting layer and atriphenyldiamine derivative for the hole transport layer. Advantages ofthe laminated structure include enhanced efficiencies of hole injectioninto the light emitting layer and of the generation of exciters, thelatter being generated from a recombination by blocking the electronsinjected from the cathode, and the enclosure of exciters generatedwithin the light emitting layer. Other known element structures fororganic EL elements such as the two-layered one cited above include athree-layered structure comprising a hole transport layer, a lightemitting layer and an electron transport layer. These laminated elementsembody many contrivances in element structure or formation methodintended to enhance the efficiency of recombination of injected holesand electrons. Also, the wavelength of emitted light can be changed byusing a different material for the light emitting layer.

However, in an organic EL element, there is a constraint to theprobability of singlet generation due to the dependence of spinstatistics at the time of carrier recombination, resulting in an upperlimit to light emitting efficiency. The level of this upper limit isknown to be about 25%. When an iridium complex is used as the dopantmaterial for the light emitting layer, luminescence from the tripletexciter of iridium arises at a high probability and this, combined withthe utilization of the singlet exciter, enables exciter generation at ahigh probability of 75 to 100%.

Incidentally, organic EL elements and the like confine light by a totalreflection effect as an optical phenomenon characteristic of them. Asthe refractive index of the light emitting layer or the transparentelectrode is higher than that of the substrate or air, light whose angleof emission is at or above a critical angle is totally reflected by thetransparent electrode/substrate interface or the substrate/airinterface, and cannot be extracted out of the substrate. Supposing thatthe refractive index of the organic layer including the emitting layeris 1.6, that of the transparent electrode 2.0 and that of the substrate1.5, the quantity of light emitted outside, namely the efficiency oflight extraction is no more than 20% or so. For this reason, the limitof energy conversion efficiency is never high, only about 5% includingthe probability of singlet generation or, even if the triple exciter isutilized, no more than 15 to 20% in total. This poses a problem not onlyto organic EL elements but also to plane-light emitting elements ingeneral, whose light emitting material discharges light.

As a method to enhance this light extraction efficiency, it is proposedin Patent Document 1 (Japanese Patent Laid-Open No. 2001-202827) toarrange a low refractive index layer between the substrate and thetransparent electrode, and this laminated structure is shown in FIG. 11.According to this disclosed method, the presence of a transparentelectroconductive film (transparent electrode layer) 302 in contact withat least one surface of a low refractive index body 301 serves toenhance the rate of extracting the light passing the low refractiveindex body 301 into the atmosphere and this enhanced rate of extractingthe light outside, and the refractive index 1.003 to 1.300 of the lowrefractive index body 301 enable the light passing the low refractiveindex body 301 to be more efficiently extracted into the atmosphere,resulting in a higher extracting ratio of light to be extracted to theoutside. Furthermore, an ultra-low refractive index dose to 1 isrealized by using silica aerogel for the low refractive index body 301.

Also, Patent Document 2 (Japanese Patent Laid-Open No. 2002-278477)discloses an invention by the same inventor in which the light emittingelement of Patent Document 1 is applied to a thin film transistor (TFT)substrate. According to this disclosed technique, a low refractive indexlayer on the other side surface of the transparent electroconductivelayer than the light emitting layer is within a range of 1.01 to 1.3.

Further, Non-Patent Document 1 (T. Tsutsui, Adv. Mater. 2001, 13, No.15, August 3, pp. 1149-1152) discloses a structure in which an organicEL element is provided over a substrate having a silica aerogel film of10 μm in thickness arranged as the low refractive index layer, a siliconoxide (SiO₂) film of 50 nm in thickness arranged over the silica aerogelfilm, and a transparent electrode (ITO) of 100 nm in thickness arrangedover the silicon oxide film. The silicon oxide film here is formed bysputtering. The reference states that the external quantum efficient ofthe disclosed structure is 1.8 times higher than that of an elementstructure having no silica aerogel film.

However, the above related arts leave room for improvement in thefollowing respects.

The structures described in Patent Documents 1 and 2, in which a lowrefractive index layer is arranged between the substrate and thetransparent electrode layer, are effective in that the efficiency oflight extraction is enhanced by collecting the light within a criticalangle, but the reflection of light by the interface between thetransparent electrode and the low refractive index layer makes theenhancement of light extraction efficiency still insufficient. When aporous silica aerogel film is used to obtain an ultra-low refractiveindex layer, the mechanical strength of the film is extremely weak.

Further, when the transparent electrode is patterned in a wet process,the etchant flowing round from the porous silica aerogel film makes itdifficult to form the prescribed pattern. Moreover, the surfaceroughness of the porous film invites inter-electrode leaks and pixelshort, giving rise to unstable light emission or failure to emit light.Thus, these techniques are still inadequate as light extraction methodsthat can be applied to organic EL elements.

The method described in Non-Patent Document 1 by which a silicon oxidefilm is formed by sputtering over a silica aerogel film of 10 μm inthickness is also inadequate in that the sputtered film does not serveto improve surface roughness of the silica aerogel film, and similarlyinvites interelectrode leaks and pixel short. Further it is difficult toform a silica aerogel film as thick as 10 μm with sufficient uniformity,and accordingly element characteristics are susceptible to fluctuation.If this method is to be applied to a display device provided with a thinfilm transistor (TFT), the 10 μm film thickness will make it difficultto form contact holes needed for connecting the pixel electrode and thesource electrode of the TFT, making the intended application impossible.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the problems of the priorart stated above and provide an optical substrate for light emittingelements excelling in light extraction efficiency, a light emittingelement using it, and a display device using the light emitting element.In particular, the invention is intended to provide a reliable lightemitting element and display device with high yield, and a displaydevice of high grade picture quality and high resolution.

An optical substrate according to the invention comprises a transparentsubstrate, a low refractive index layer, whose refractive index is lowerthan that of the transparent substrate, disposed over the transparentsubstrate, and a solgel film disposed over the low refractive indexlayer.

A light emitting element according to the invention comprises atransparent substrate, a low refractive index layer, whose refractiveindex is lower than that of the transparent substrate, disposed over thetransparent substrate, a solgel film disposed over the low refractiveindex layer, a first electrode over the solgel film, a light emittinglayer over the first electrode, and a second electrode over the lightemitting layer.

A display device according to the invention comprises a light emittingelement and means for driving the light emitting element, which aredisposed over the transparent substrate, wherein the light emittingelement comprises a low refractive index layer, whose refractive indexis lower than that of the transparent substrate, disposed over thetransparent substrate, a solgel film disposed over the low refractiveindex layer, a first electrode over the solgel film, a light emittinglayer over the first electrode, and a second electrode over the lightemitting layer. The plurality of light emitting elements in this displaydevice can be formed in a matrix shape.

According to the invention, the light extraction efficiency issignificantly improved by the solgel film disposed over the lowrefractive index layer. The solgel film serves to ease the surfaceroughness of the low refractive index layer, reduces the diffusereflection arising on the interface between the low refractive indexlayer and the transparent electrode, and can thereby significantlyimprove the light extraction efficiency.

According to the invention, the refraction index of the low refractiveindex layer can be set between 1.003 and 1.400.

According to the invention, silica aerogel can suitably used for the lowrefractive index layer.

According to the invention, materials that have an Si—O—C structure canbe suitably used for the solgel film.

According to the invention, materials that have in its infraredabsorption spectrum an absorption of 1107±2 cm⁻¹ in wave numberattributable to Si—O—C can be used for the solgel film.

According to the invention, materials that have in its infraredabsorption spectrum an absorption A of 1107±2 cm⁻¹ in wave numberattributable to Si—O—C and an absorption B of 1070±2 cm⁻¹ in wave numberattributable to Si—O—Si, and the intensity ratio A/B between theabsorption A and the absorption B ranging from 0.5 to 1.0 can besuitable used for the solgel film.

According to the invention, a relationship between the surface roughnessof the low refractive index layer represented by Ra1 and that of thesolgel film over the low refractive index layer represented by Ra2 canbe Ra1>Ra2.

According to the invention, the refractive index of the solgel film canbe greater than that of the low refractive index layer.

According to the invention, the thickness of the solgel film can bebetween 0.05 and 1.0 μm.

According to the invention, the solgel film can be formed by using aprecursor having an Si—H group.

According to the invention, a plastic substrate can be suitably used asthe transparent substrate.

According to the invention, a barrier layer can be further disposed overthe solgel film and the first electrode is disposed over the barrierlayer.

According to the invention, the transparent substrate can be a substrateprovided with a thin film transistor.

The light emitting element according to the invention can be so composedthat light emission from the light emitting layer be monochromatic.Further this monochromatic light can be either white light or bluelight.

The light emitting element according to the invention can be applied tomany different types of optical elements including light emitting diodeelements, plasma display elements and so forth besides organic ELelements and inorganic EL elements.

The display device according to the invention can be a TFT displaydevice provide with an electronic circuit comprising a thin filmtransistor (TFT) and other elements as the driving means.

The display device according to the invention can have a configurationhaving a level gap easing film for easing a convex attributable to theelectronic circuit comprising a TFT and other elements.

The display device according to the invention can be so configured as tokeep the film thickness of the low refractive index layer at or below 4μm.

A method of manufacturing an optical substrate according to theinvention comprises the steps of disposing a low refractive index layerover a transparent substrate; coating solgel film over the lowrefractive index layer; and increasing the density of the solgel film byirradiation with ultraviolet rays after drying. The light emittingelement can be provided over this solgel film. The low refractive indexlayer can be disposed over the transparent substrate provided withdriving means.

The present invention can provide the following advantages.

A first advantage is to reduce with the solgel film disposed over thelow refractive index layer the diffuse reflection arising on theinterface of the low refractive index layer, and thereby significantlyimprove the light extraction efficiency of the optical substrate. Theuse of this optical substrate makes it possible to provide a lightemitting element and a display device of high light emitting efficiency,in other words high luminance, yet consuming less electric power.

A second advantage is that, as the surface roughness of the lowrefractive index layer is eased by the solgel film, inter-electrodeleaks or pixel short can be restrained even where a light emitting layerof a very thin film thickness is used such as in the organic EL element,with the result that it is possible to provide a light emitting elementand a display device of high luminance, yet consuming less electricpower, and moreover excelling in production yield and reliability.

A third advantage is that a level gap easing layer for the eliminationof the convex attributable to the means of driving the light emittingelement, such as a TFT, makes it possible to form the low refractiveindex layer free from unevenness of film thickness and cracking, withthe result that it is possible to provide a display device of highluminance, yet consuming less electric power, excelling in productionyield, in reliability and moreover in picture quality.

A fourth advantage is that because of the possibility to obtain a lightemitting element manifesting a high light emitting efficiency even witha low refractive index layer of a film thickness of 4 μm or below, even1 μm or below, the contact hole size can be kept very small, with theresult that it is possible to provide a display device of highluminance, yet consuming less electric power, excelling in productionyield, in reliability and in picture quality with the additionalbenefits of making available very fine contact holes in a highproportion.

A fifth advantage is that the presence of an Si—H group in the precursorto the solgel film means that optical reaction by irradiation withultraviolet rays can be used for setting the solgel film. As this makespossible a low temperature process, which can be applied to plasticsubstrates not very resistant to heat, it is possible to provide alight, thin and flexible optical substrate, and a light emitting elementand a display device with a high light emitting efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of an example of optical substrateaccording to the present invention;

FIG. 2 shows a sectional view of another example of optical substrateaccording to the invention;

FIG. 3 is a process diagram charting the manufacturing method of theoptical substrate according to the invention;

FIG. 4 shows an infrared absorption spectrum before and after thephotoreaction of a solgel film;

FIG. 5 shows a sectional view of an example of optical element accordingto the invention;

FIG. 6 shows a sectional view of another example of optical elementaccording to the invention;

FIG. 7 shows a sectional view of another example of optical elementaccording to the invention;

FIG. 8 shows a sectional view of another example of display deviceaccording to the invention;

FIG. 9 shows a sectional view of another example of optical elementaccording to the invention;

FIG. 10 shows a sectional view of another example of optical elementaccording to the invention; and

FIG. 11 shows a sectional view of a conventional optical substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described indetail below with reference to accompanying drawings.

First Embodiment

FIG. 1 shows a sectional view of an optical substrate as a firstembodiment of the invention. Thus, an optical substrate 10 according tothe invention is provided with a low refractive index layer 2 on oneface of a transparent substrate 1 and a solgel film 3 on the lowrefractive index layer 2.

FIG. 2 shows a sectional view of a substrate configured by disposing afirst electrode layer 20 in contact with the upper face of the opticalsubstrate of FIG. 1.

This optical substrate, mounted with a light emitting area above thesolgel film 3, is used as the substrate for a light emitting element.The solgel film 3 alleviates the surface roughness of the low refractiveindex layer 2, reduces the diffuse reflection arising on the interfacebetween the low refractive index body 301 and the transparent electrode302, where no solgel film is present as seen in FIG. 11, and can therebyserve to significantly improve the light extraction efficiency. In otherwords, two interfaces including that between the low refractive indexlayer 2 and the solgel film 3 and that between the solgel film 3 and thefirst electrode layer 20 constitute an important element pertaining tothis embodiment of the invention.

Constituent parts of the optical substrate pertaining to this embodimentwill be described in detail below.

The transparent substrate 1 is used as the light extraction substrate ofthe light emitting element. It transmits at least some wavelengths inthe visible radiation range. It is sufficient for the transparentsubstrate 1 in this embodiment to transmit lights of at least somewavelengths between 400 and 800 nm, and its material may be eitherinorganic or organic. Available inorganic materials include glass, andorganic materials, plastics. Among the usable types of glass includeoptical glasses such as fused silica, non-alkali glass, soda glass orheavy flint glass. Plastics usable for the purpose include engineeringplastics such as polyether sulfone (PES) or polyethylene terephthalate(PET). The appropriate refractive index of the transparent substrate 1ranges from 1.4 to 2.1. The transparent substrate 1 may be coated with abarrier layer to restrain the permeation of moisture and oxygen. It isdesirable for the thickness of the transparent substrate 1, though notlimited to, to be 0.1 to 2.0 mm from a practical point of view.

A transparent material lower than the transparent substrate 1 inrefraction index is used for the low refractive index layer 2. Like thetransparent substrate, this layer may pass lights of at least somewavelengths between 400 and 800 nm. Its desirable refractive indexranges from 1.003 to 1.400. There is a limit to keep the refractiveindex low, and 1.003 is considered the practical limit. The mostpreferable material for the low refractive index layer is silicaaerogel.

Silica aerogel can be produced by drying a gelatinous compound in a wetstate, having a silica frame obtained by hydrolyzing alkoxysilane andpolymerizing the hydrolyzed product, in the presence of alcohol or someother solvent in a supercritical state beyond the critical point of thissolvent.

It is sufficient for the solgel film 3, like the transparent substrate 1and the low refractive index layer 2, to pass lights of at least somewavelengths between 400 and 800 nm. Herein, as a sol solution made of analkoxide of silicon or a metal such as titanium, water, acid, alcoholand the like can be used, but an silicon-based alkoxide is preferable inview of a degree of transparency and handling ease.

FIG. 3 shows one example of manufacturing method of the opticalsubstrate of this embodiment of the invention. FIG. 3(A) shows a statein which the low refractive index layer 2 is disposed over thetransparent substrate 1. Beginning with this state, the low refractiveindex layer 2 is coated with a solgel film as shown in FIG. 3(B), thesolvent is removed by drying, and a precursor 3′ to the solgel film isthereby obtained. Then, the precursor 3′ to the solgel film isirradiated with ultraviolet rays 8 to accelerate reactions in the solgelfilm, such as condensation. The solgel film 3, which is less in filmthickness, in other words is denser, than the precursor 3′ to the solgelfilm is obtained as shown in FIG. 3(C). It is also possible here to heatthe precursor 3′ when it is irradiated with ultraviolet rays. The heatedambiance might further increase the density. The light emitting elementand the display device embodying the invention in this mode can bemanufactured by a similar method. Ultraviolet rays of any wavelength canbe used if they can be absorbed by the precursor 3′ to the solgel film.An effective source for the ultraviolet rays can be selected from axenon lamp, xenon mercury lamp, mercury lamp, excimer lamp, excimerlaser, YAG laser and so forth. An excimer lamp would be particularlyeffective for the optical substrate according to the invention, andparticularly preferable ones include a Xe₂ lamp of 172 nm in wavelength,a Kr₂ lamp of 146 nm in wavelength and an Ar₂ lamp of 126 nm inwavelength.

FIG. 4 shows an infrared absorption spectrum before and after thephotoreaction to form the solgel film 3, which in this case is made of asilicon-based alkoxide. In FIG. 4(A), A represents the absorption peakof Si—H of 826±2 cm⁻¹; B, that of Si—O—Si of 1070±2 cm⁻¹; C, that ofSi—C of 1107±2 cm⁻¹; and D, that of Si—H of 2360±2 cm⁻¹. FIG. 4(B) showsan expanded view of the area in FIG. 4(A) where the wave number is from1000 to 1200 cm⁻¹. The solgel film 3 would usually be set by anphotoreaction or a thermal reaction; comparison of the absorption peaksbefore and after the photoreaction due to irradiation with ultravioletrays earlier described with reference to the manufacturing method,reveals that the peak of 826 cm⁻¹ in wave number attributable to Si—H isfound significantly lower after the photoreaction. This can be explainedby the fact that the precursor to the solgel film has a high content ofhighly optically reactive Si—H groups, and these Si—H groups are almosteliminated by the photoreaction due to irradiation with ultravioletrays. However, the presence of any Si—H group in the solgel film afterirradiation with ultraviolet rays poses no problem.

By utilizing this photoreaction due to irradiation with ultravioletrays, a low-temperature setting process is made possible. This enables amaterial poor in temperature-tolerance, such as plastics, for thetransparent substrate, and thereby contributes to expand the scope ofoptions for the substrate material. As a result, it is made possible toeasily obtain flexible light-weight optical substrate that can bereduced in thickness and light emitting elements highly efficient inlight extraction using such optical substrates. Even for a substrate inwhich a thin film transistor (TFT) as shown in FIG. 8 is formed by alow-temperature process, there are advantages of reducing thedeterioration of TFT characteristics and reliability. As a result, it ismade easier to provide a light-weight and flexible display devicepermitting a reduction in thickness, which also excels in luminance,quality and reliability.

Referring again to FIG. 4, there remains an absorption peak of Si—O—Cafter the reaction of the solgel film 3. When a silica aerogel film isused as the low refractive index layer, since there also is anabsorption peak of Si—O—C in the silica aerogel film, an advantage ofhigh chemical affinity is achieved by using a silicon-based alkoxide forthe solgel film 3. It is preferable for the absorption peak of Si—O—C tohave an intensity ratio (absorbance ratio: Si—O—C/Si—O—Si) of 0.5 to 1.0to the absorption peak of Si—O—Si. A greater value of this intensityratio than 1.0 means an insufficient condensing reaction of the solgelfilm.

The optical substrate pertaining to this embodiment of the invention isfabricated by successively stacking the low refractive index layer 2 andthe solgel film 3 over the transparent substrate 1. The light emittingelement pertaining to this embodiment is fabricated by stacking at leasta first electrode 20 and a light emitting layer over the opticalsubstrate. For this reason, if the surface of the transparent substrate1 is uneven, the unevenness will affect the first electrode 20 and thelight emitting layer, inviting current leaks and short circuitingbetween electrodes. Therefore, it is preferable to use a flattransparent substrate 1. However, many of the materials available forthe low refractive index layer 2 are porous in film quality, as istypically found in silica aerogel, and not insignificant in surfaceroughness. Then, where the surface roughness of the low refractive indexlayer 2 is represented by Ra1 and that of the solgel film 3 over the lowrefractive index layer 2 by Ra2, a relationship ofRa1>Ra2should hold between them. The surface roughness here is defined in termsof the center line average roughness. Therefore, it is possible enhancethe efficiency of light extraction by smoothening the base of the firstelectrode 20 to restrain current leaks and short circuiting which occurbetween electrodes and, at the same time reducing the diffuse reflectionarising on the two interfaces, one between the low refractive indexlayer 2 and the solgel film 3 and the other between the solgel film 3and the first electrode layer 20.

Further, in order to ease the unevenness of the low refractive indexlayer 2 with the solgel film 3 and reduce the surface roughness of thesolgel film 3, finer film quality than the porous one is required.Generally speaking, finer the film quality, the greater the refractiveindex. Therefore, it is preferable for the refractive index of thesolgel film 3 to be greater than that of the low refractive index layer.

Though the appropriate thickness of the solgel film 3 ranges from 0.05to 1.0 μM, it is more preferable for the thickness to be between 0.05and 0.5 μM and still more preferable to be between 0.05 and 0.3 μm. Ifit is less than 0.05 μm, it will be difficult to ease the unevenness ofthe low refractive index layer 2, namely to reduce Ra2 mentioned above.If the thickness is greater than 1.0 μm thickness irregularities orcracks due to factors during film formation are will be more likely tooccur, inviting destabilization of optical characteristics.

In this embodiment, the use of a transparent electroconductive material,such as indium tin oxide (ITO) alloy or antimony-doped tin oxide (ATO)for the solgel film 3 makes possible using it as an auxiliary electrodeto reduce the resistance of the first electrode layer 20.

Second Embodiment

This embodiment of the invention is an example in which the opticalsubstrate described with reference to the first embodiment is used foran organic electroluminescence (EL) element.

FIG. 5 shows a sectional view of an example of organic EL elementpertaining to this embodiment of the invention. The first electrode 20,an organic light emitting layer 50 and a second electrode 30 aresuccessively provided over the optical substrate 10. The organic lightemitting layer 50 here consists of one or more layers. Its possiblestructures having a plurality of layers include a two-layered structureconsisting of a hole transport layer and a light emitting layer or alight emitting layer and an electron transport layer; a three-layeredstructure consisting of a hole transport layer, a light emitting layerand an electron transport layer; a four layered structure consisting ofa hole injection layer, a hole transport layer, a light emitting layerand an electron transport layer; and a five-layered structure consistingof a hole injection layer, a hole transport layer, a light emittinglayer, a hole blocking layer and an electron transport layer. Thecathode may be provided on the light emitting layer side with a bufferlayer of a low work function metal, a fluoride or the like.Incidentally, the organic EL element in this embodiment can either be alow molecular type or a high molecular type. The optical substrate ofthe organic EL element is so arranged in this embodiment that the lightemitted from the light emitting layer pass the optical substrate.

A wide variety of materials are available for use in hole transport. Inspecific terms, there are diamine derivatives including N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (abbreviated to TPD)and N,N′-diphenyl-N,N′-bis(α-naphthyl)-1,1′-biphenyl-4,4′-diamine(abbreviated to α-NPD);4,4′,4″-tris(3-methylphenylphenylamino)-triphenylamine; and star bursttype molecules.

Materials for electron transport are also available in a wide variety.Specifically, there are tris(8-quinolinol) aluminum complex (Alq3);oxadiazole derivatives including2-(4-biphenylyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazol andbis{2-(4-t-butylphenyl)-1,3,4-oxadiazol}-m-phenylene; triazolederivatives; and quinolinol-based metallic complexes.

Available light emitting materials include, for instance,tris(8-quinolinol) aluminum complex (Alq3), bisdiphenylvinylbiphenyl(BDPVBi), 1,3-bis(p-t-butylphenyl)-1,3,4-oxadiazolil) phenyl (OXD-7),N,N′-bis(2, 5-di-t-butylphenyl) perylenetetracarboxylic diimide (BPPC)and 1,4 bis(p-tolyl-p-methylstyrylphenyl) naphthalane. A layer formed bydoping a charge transport material with a fluorescent material can alsobe used as the light emitting material. For instance, it is possible touse a layer formed by doping a quinolinol metal complex, such as Alq3mentioned above with4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM), aquinacridon derivative such as 2,3-quinacridon [7], a coumarinderivative such as 3-(2′-benzothiazole)-7-diethylaminocoumarin,perylene, dibenzonaphihacene or benzopyrene; a layer formed by doping anelectron transport material consisting of a bis(2-methyl-8-hydroxyquinoline)-4-phenylphenol-aluminum complex with acondensed polycyclic aromatic compound such as perylene; or a layerformed by doping a hole transport material consisting of 4,4′-bis(m-tolylphenylamino) biphenyl (TPD) with rubrene or the like.

In the element shown in FIG. 5, the first electrode 20 performs the roleof injecting holes into the hole transport layer or the light emittinglayer, and should preferably have a work function of 4.5 eV or more.Available materials for the first electrode 20 used in this embodimentinclude transparent electroconductive materials such as indium tin oxide(ITO) alloys and indium zinc oxide (IZO) alloys. The second electrode 30performs the role of injecting electrons into the electron transportlayer or the light emitting layer, and should preferably have a low workfunction. The material for a cathode 120 in specific terms may be,though not limited to, indium, aluminum, magnesium, magnesium-indiumalloy, magnesium-aluminum alloy, aluminum-lithium alloy,aluminum-scandium-lithium alloy, magnesium-silver alloy or the like.

Incidentally, the organic EL element of this embodiment can as well beeither passively driven or actively driven with an active element, suchas a thin film transistor (TFT), being added.

The methods of forming the layers constituting the organic EL element ofthis embodiment are not limited to any specific ones, but can beappropriately selected from known ones, such as vacuum deposition,molecular beam epitaxy (MBE), and coating methods including dipping in asolution dissolved in a solvent, spin coating, casting, bar coating androll coating.

In this embodiment, the optical substrate 10 can enhance lightextraction efficiency in the visible radiation range without dependingon the wavelength of the emitted light. Therefore, a high-efficiencyarea color element or a high-efficiency full color element can beobtained by using materials independently emitting red, green and bluecolors as the organic light emitting layer 50; a high-efficiency colorfilter type organic EL element using a white light emitting layer 70shown in FIG. 6 or a high-efficiency color conversion type organic ELelement using a blue light emitting layer 80 shown in FIG. 7 can be alsoobtained. The white light emitting layer or the blue light emittinglayer may be configured of a plurality of layers like the ones describedabove.

The color filter type organic EL element of FIG. 6 is provided with ared color filter 220, a green color filter 221 and a blue color filter222 on the other face of the optical substrate 10 comprising thetransparent substrate 1, the low refractive index layer 2 and the solgelfilm 3 than the low refractive index layer 2 side. It has aconfiguration in which the first electrode 20, a white light emittinglayer 70 and the second electrode 30 are successively stacked over thesolgel film 3. The white emitted light 210 from the white light emittinglayer 70 is separated into a red emitted light 200, a green emittedlight 201 and a blue emitted light 202 by the red color filter 220, thegreen color filter 221 and the blue color filter 222, respectively. Thisenables a high luminance full color element to be obtained.

The color conversion type organic EL element shown in FIG. 7 is providedwith a red conversion filter 230 and a green conversion filter 231 onthe other face of the optical substrate 10 comprising the transparentsubstrate 1, the low refractive index layer 2 and the solgel film 3 thanthe low refractive index layer 2 side. It has a configuration in whichthe first electrode 20, the blue light emitting layer 80 and the secondelectrode 30 are successively stacked over the solgel film 3. The blueemitted light 202 from the blue light emitting layer 80 is convertedinto the red emitted light 200 and the green emitted light 201 by redconversion filter 230 and the green conversion filter 231, respectively.The blue emitted light 202 here can be emitted either as it is or afterbeing further improved in color purity by a color filter providedspecifically for this purpose.

As hitherto described, the light emitting element of this embodiment ofthe invention can provide a full color organic EL element having a highlight emitting efficiency.

Whereas FIG. 5 through FIG. 7 shows element structures regarding thelight emitting element, what is used in an actual display devicecomprises a light emitting element and drive means, such as a thin filmtransistor (TFT), provided over the transparent substrate 1.

FIG. 8 shows a sectional view of another example of organic EL displaydevice of this embodiment. The low refractive index layer 2 and thesolgel film 3 are disposed on one face of a TFT-formed substrate 110,and the first electrode 20, the organic light emitting layer 50 and thesecond electrode 30 matching each pixel are stacked in that order overthe solgel film 3. Since the light emitting layer 50 is a very thinfilm, it is provided with an edge protecting layer 109 to prevent anyunevenness of the edge of the first electrode 20 from invitinginter-electrode short circuiting with the second electrode 30. In theTF-formed substrate 110, reference numeral 100 denotes a pixel-drivingTFT circuit unit; 101, polysilicon; 104, a gate electrode; 105, a gateoxide film; 102, a drain electrode; 103, a source electrode; 106, afirst inter-layer film; 107, a second inter-layer film; and 108, a levelgap easing film. The pixel-driving TFT circuit unit 100, which is anelement to control light emission by the light emitting layer 50, isarranged at each of the intersections between gate lines and data lines(not shown) extending in mutually orthogonal directions. It is possibleto dispose a plurality of pixel-driving TFT circuit units depending onthe driving system.

In the display device of this embodiment, the level gap easing film 108is disposed outside the area of the pixel-driving TFT circuit unit 100,in other words an area L matching the light emitting layer 50. If nolevel gap easing film were disposed, the area L would be made concave bythe pixel-driving TFT circuit unit 100. This concave would gather liquidin the process of wet-coating the low refractive index layer 2 or thesolgel film 3, giving rise to unevenness of film thickness and crackingand thereby inviting poor picture quality. This problem is solved byproviding the level gap easing film 108.

In the optical substrate or the light emitting element of thisembodiment, the low refractive index layer 2 can have any desired filmthickness, but in the display device of the embodiment a film thicknessof greater than 4 μm for the low refractive index layer 2 would make itdifficult to form contact holes for connecting the source electrode 103and the first electrode 20. In particular, a very fine display device of130 ppi or above, where the distances between pixels is short, requiresa contact hole size of less than 4 μm in order to achieve a highproportion of holes. For this reason, it is preferable for the filmthickness of the low refractive index layer 2 to be not greater than 4μm and, more preferably, 1 μm or less. With a view to enable the lowrefractive index layer to be fully effective, its film thickness shouldbe not less than 0.1 μm.

As described so far, the display device embodying the invention in thismode can be a full color display device of high luminance, yet consumingless electric power.

Third Embodiment

To illustrate this embodiment of the invention, an example having abarrier layer between the solgel film and the first electrode in thelight emitting element described with reference to the second embodimentwill be cited.

FIG. 9 shows a sectional view of one example of organic EL element ofthis embodiment A barrier layer 4 is arranged over the optical substrate10, and the first electrode 20, the organic light emitting layer 50 andthe second electrode 30 are successively disposed over the barrier layer4.

Generally speaking, a solgel film, though excelling in flatteningfunction, has a characteristic of high water absorbance, which is due tothe chemical structure of the film. Many of the organic materialsavailable for organic EL elements are readily deteriorated by moisture.For this reason, it is necessary to dehydrate the optical substrate byheating at high temperature or otherwise at the time of composing theorganic material.

The barrier layer 4 has a function to confine moisture discharged fromthe solgel film 3. By providing the barrier layer 4, this dehydrationcan be either totally dispensed with or at least the hydration processcan be shortened in time. This enables a high luminance full colororganic EL element to be obtained at low cost.

Fourth Embodiment

To illustrate this embodiment of the invention, an example in which theoptical substrate described with reference to the first embodiment isused for an inorganic electroluminescence (EL) element will bedescribed.

FIG. 10 shows a sectional view of one example of inorganic EL element ofthis embodiment. The first electrode 20, an insulating layer 40, aninorganic light emitting layer 60, another insulating layer 40 and thesecond electrode 30 are successively disposed over the optical substrate10. In the inorganic EL element of this embodiment, the opticalsubstrate is so arranged that the light emitted from the light emittinglayer pass the optical substrate. Incidentally, the inorganic EL elementof this embodiment can be fabricated by using known materials.

In this embodiment, the optical substrate 10 can enhance lightextraction efficiency in the visible radiation range without dependingon the wavelength of the emitted light. Therefore, a high-efficiencyarea color element or a high-efficiency full color element can beobtained by using materials independently emitting red, green and bluecolors as the inorganic light emitting layer 60; a high-efficiency colorfilter type inorganic EL element using a white light emitting layer or ahigh-efficiency color conversion type inorganic EL element using a bluelight emitting layer can be also obtained as described with reference tothe second embodiment. This enables a high luminance full colorinorganic EL element to be obtained.

EXAMPLES

The present invention will be described with reference to examplesthereof. Incidentally, the invention is not limited to the followingexamples, but can be implemented with various appropriate modifications.

In the following examples, the light emitting characteristics of theorganic EL element were measured with a luminance meter (SR-3, a productof Topcon Technohouse Corporation) arranged in the direction of thenormal direction of the substrate at a condensing angle of 0.1 degree.The area of each light emitting portion of the organic EL element was0.4 cm², and a constant current was applied to this element to measurethe current-luminance efficiency at a current density of 1 mA/cm² andthe element voltage working between the first electrode and the secondelectrode. Each element had four light emitting portions, and themeasurement was taken as the average of the four points. Data from anylight emitting portion where inter-electrode leaks or pixel shortoccurred were disregarded.

In the following examples and comparative examples, green, red and bluelight emitting layers are used over a prescribed optical substrate.Comparative Example 1 will be used as the reference for green,Comparative Example 2 for red, and Comparative Example 3 for blue; thecurrent-luminance efficiency (light emitting efficiency) counts will berelative values against a reference level of 100; and the elementvoltages will be relative voltage differences from the reference. Themeasured results are put together in Table 1.

Example 1

A non-alkali glass (OA-10, a product of Nippon Electric Glass Co., Ltd.)of 0.7 mm in thickness and 1.52 in refraction index was selected for thetransparent substrate, and a silica aerogel film of 1.25 in refractionindex and 0.85 μm in thickness was formed by spin coating as the lowrefractive index layer over one face of this transparent substrate. Thesurface roughness Ra1 of the low refractive index layer then was 8 nm.Next, a film of an alkoxide of silicon (T12-800, a product of Tokyo OhkaKogyo Co., Ltd.) of 1.50 in refraction index and 0.1 μm in thickness wasformed by spin coating as the solgel film over the low refractive indexlayer to fabricate an optical substrate. In forming the solgel film,irradiation with ultraviolet rays of 600 mJ/cm² in cumulative luminousenergy was carried out. The surface roughness Ra2 of the solgel filmthen was 5 nm, smaller than Ra1.

Then, ITO was formed into a film by sputtering to have a sheetresistance of 15 Ω/□ or less as the first electrode over the opticalsubstrate, and the film was wet-etched with an etchant consisting ofhydrochloric acid, nitric acid and water to obtain a prescribed pattern.The ITO film was 100 nm thick and had a refractive index of 1.78. Next,a film of α-NPD (N,N′-diphenyl-N,N′-bis(α-naphthyl)-1,1′-biphenyl-4,4′-diamine) was formed by vacuum deposition as the holeinjection/transport layer over the ITO film to a thickness of 50 nm.Then, to form the light emitting layer, Alq3 (tris(8-quinolinol)aluminum complex) was co-deposited with a dopant of quinacridon (in adoping concentration of 4 wt %) to a thickness of 25 nm. Next, a film ofAlq3 was formed by vacuum deposition as the electron transport layer toa thickness of 35 nm. Finally, Al and U were co-deposited to a thicknessof 30 nm, followed by deposition only of Al to 40 nm to form the cathodelayer; a green light-emitting organic EL element was formed. As a resultthe current-luminance efficiency was found to be 184, and the elementvoltage difference, 0.4 V.

Example 2

A red light-emitting organic EL element was formed under the sameconditions as Example 1 except that Alq3 (tris(8-quinolinol) aluminumcomplex) was co-deposited with a dopant of dicyanomethylenepyran (DCM,in a doping concentration of 5 wt %) as the light emitting layer to athickness of 25 nm. As a result, the current-luminance efficiency wasfound to be 166, and the element voltage difference, 0.3 V.

Example 3

A blue light-emitting organic EL element was formed under the sameconditions as Example 1 except that 4,4′-bis (2,2-diphenylvinyl)biphenyl was co-deposited as the light emitting layer to a thickness of35 nm. As a result, the current-luminance efficiency was found to be182, and the element voltage difference, 0.3 V.

Example 4

A non-alkali glass (OA-10, a product of Nippon Electric Glass Co., Ltd.)of 0.7 mm in thickness and 1.52 in refraction index was selected for thetransparent substrate, and a silica aerogel film of 1.25 in refractionindex and 0.85 μm in thickness was formed by spin coating as the lowrefractive index layer over one face of this transparent substrate. Thesurface roughness Ral of the low refractive index layer then was 8 nm.Next, a film of an alkoxide of silicon (T12-800, a product of Tokyo OhkaKogyo Co., Ltd.) of 1.50 in refraction index and 0.1 μm in thickness wasformed by spin coating as the solgel film to fabricate an opticalsubstrate. In forming the solgel film, irradiation with ultraviolet raysof 600 mJ/cm² in cumulative luminous energy was carried out. The surfaceroughness Ra2 of the solgel film then was 5 nm, smaller than Ra1.

Then, a silicon oxide film of 1.5 in refractive index and 0.05 μm inthickness was formed as a barrier layer by CVD over the opticalsubstrate; ITO was formed into a film by sputtering to have a sheetresistance of 15 Ω/□ or less as the first electrode over the barrierlayer, and the film was wet-etched with an etchant consisting ofhydrochloric acid, nitric acid and water to obtain a prescribed pattern.The ITO film was 100 nm thick and had a refraction index of 1.78. Next,a film of α-NPD (N,N′-diphenyl-N,N′-bis(α-naphthyl)-1,1′-biphenyl-4,4′-diamine) was formed by vacuumdeposition as the hole injection/transport layer over the ITO film to athickness of 50 nm. Then, to form the light emitting layer, Alq3(tris(8-quinolinol) aluminum complex) was co-deposited with a dopant ofquinacridon (in a doping concentration of 4 wt %) to a thickness of 25nm. Next, a film of Alq3 was formed by vacuum deposition as the electrontransport layer to a thickness of 35 nm. Finally, Al and U wereco-deposited to a thickness of 30 nm, followed by deposition only of Alto 40 nm to form the cathode layer; a green light-emitting organic ELelement was formed. As a result the current-luminance efficiency wasfound to be 170, and the element voltage difference, 0.1 V.

Example 5

A red light-emitting organic EL element was formed under the sameconditions as Example 4 except that Alq3 (tris(8-quinolinol) aluminumcomplex) was co-deposited with a dopant of dicyanomethylenepyran (DCM,in a doping concentration of 5 wt %) as the light emitting layer to athickness of 25 nm. As a result, the current-luminance efficiency wasfound to be 160, and the element voltage difference, 0 V.

Example 6

A blue light-emitting organic EL element was formed under the sameconditions as Example 4 except that 4,4′-bis (2,2-diphenylvinyl)biphenyl was co-deposited as the light emitting layer to a thickness of35 nm. As a result, the current-luminance efficiency was found to be171, and the element voltage difference, 0 V.

Comparative Example 1

A green light emitting element was formed under the same conditions asExample 1 by using a transparent substrate in which neither a lowrefractive index layer nor a solgel film was formed. Thecurrent-luminance efficiency and the element voltage of this comparativeexample were used as references for the green light emitting element ofeach example.

Comparative Example 2

A red light emitting element was formed under the same conditions asExample 2 by using a transparent substrate in which neither a lowrefractive index layer nor a solgel film was formed. Thecurrent-luminance efficiency and the element voltage of this comparativeexample were used as references for the red light emitting element ofeach example.

Comparative Example 3

A blue light emitting element was formed under the same conditions asExample 3 by using a transparent substrate in which neither a lowrefractive index layer nor a solgel film was formed. Thecurrent-luminance efficiency and the element voltage of this comparativeexample were used as references for the blue light emitting element ofeach example.

Comparative Example 4

A non alkali glass (OA-10, a product of Nippon Electric Glass Co., Ltd.)of 0.7 mm in thickness and 1.52 in refraction index was selected for thetransparent substrate, and a silica aerogel film of 1.25 in refractionindex and 0.85 μm in thickness was formed by spin coating as the lowrefractive index layer over one face of this transparent substrate. Thesurface roughness Ra1 of the low refractive index layer then was 8 nm.Next, a silicon oxide film of 1.50 in refractive index and 0.1 μm inthickness was formed by sputtering over the low refractive index layerto fabricate an optical substrate. The surface roughness Ra2 of thesilicon oxide film then was 8 nm, no different from Ra1. Over thisoptical substrate, a green light emitting organic EL element was formedunder the same conditions as Example 1. As a result, thecurrent-luminance efficiency was found to be 120, and the elementvoltage difference, 0.1 V. Out of the four light emitting portions, onesuffered significant inter-electrode leaks, and gave no prescribed lightemission.

Comparative Example 5

A non-alkali glass (OA-10, a product of Nippon Electric Glass Co., Ltd.)of 0.7 mm in thickness and 1.52 in refraction index was selected for thetransparent substrate, and a silica aerogel film of 1.25 in refractionindex and 0.85 μm in thickness was formed by spin coating as the lowrefractive index layer over one face of this transparent substrate. Thesurface roughness Ra1 of the low refractive index layer then was 8 nm.Next, a silicon oxide film of 1.50 in refractive index and 0.1 μM inthickness was formed by sputtering over the low refractive index layerto fabricate an optical substrate. The surface roughness Ra2 of thesilicon oxide film then was 8 nm, no different from Ra1. Over thisoptical substrate, a red light emitting organic EL element was formedunder the same conditions as Example 2. As a result, thecurrent-luminance efficiency was found to be 116, and the elementvoltage difference, 0 V. Out of the four light emitting portions, twosuffered significant inter-electrode leaks, and gave no prescribed lightemission.

Comparative Example 6

A non-alkali glass (OA-10, a product of Nippon Electric Glass Co., Ltd.)of 0.7 mm in thickness and 1.52 in refraction index was selected for thetransparent substrate, and a silica aerogel film of 1.25 in refractionindex and 0.85 im in thickness was formed by spin coating as the lowrefractive index layer over one face of this transparent substrate. Thesurface roughness Ra1 of the low refractive index layer then was 8 nm.Next, a silicon oxide film of 1.50 in refractive index and 0.1 μm inthickness was formed by sputtering over the low refractive index layerto fabricate an optical substrate. The surface roughness Ra2 of thesilicon oxide film then was 8 nm, no different from Ra1. Over thisoptical substrate, a blue light emitting organic EL element was formedunder the same conditions as Example 3. As a result, thecurrent-luminance efficiency was found to be 118, and the elementvoltage difference, 0.1 V. Out of the four light emitting portions, onesuffered significant inter-electrode leaks, and gave no prescribed lightemission.

Comparative Example 7

A non-alkali glass (OA-10, a product of Nippon Electric Glass Co., Ltd.)of 0.7 mm in thickness and 1.52 in refraction index was selected for thetransparent substrate, and a silica aerogel film of 1.25 in refractionindex and 0.85 μm in thickness was formed by spin coating as the lowrefractive index layer over one face of this transparent substrate; theoptical substrate was formed. To was formed into a film by sputtering tohave a sheet resistance of 15 Ω/□ or less as the first electrode overthis optical substrate, and the film was wet-etched with an etchantconsisting of hydrochloric acid, nitric acid and water to obtain aprescribed pattern. However, the etchant flowed round even into the areanot to be etched, which was due to the porous quality of the silicaaerogel film constituting the low refractive index layer, and theprescribed ITO pattern was not obtained. TABLE 1 Light ElementNon-emitting Remarks Color of emitting voltage portions (1st electrodelight efficiency difference (out of four) base) Example 1 Green 184 0.4V 0 Transparent Example 2 Red 166 0.3 V 0 substrate/low Example 3 Blue182 0.3 V 0 refractive index layer/solgel film Example 4 Green 170 0.1 V0 Transparent Example 5 Red 160 0.0 V 0 substrate/low Example 6 Blue 1710.0 V 0 refractive index layer/solgel film/barrier layer ComparativeGreen 100 — 0 Only Example 1 (Reference) (Reference) transparentComparative Red 100 — 0 substrate Example 2 (Reference) (Reference)Comparative Blue 100 — 0 Example 3 (Reference) (Reference) ComparativeGreen 120 0.1 V 1 Transparent Example 4 substrate/low Comparative Red116 0.0 V 2 refractive index Example 5 layer/sputtered Comparative Blue118 0.1 V 1 silicon oxide Example 6 film Comparative — EvaluationEvaluation 4 Transparent Example 7 impossible impossible substrate/lowrefractive index layer

The present invention hitherto described can be utilized for lightemitting elements for use in organic EL elements, inorganic EL elements,light emitting diode elements, plasma display elements and so forth; anddisplay devices.

1. An optical substrate comprising: a transparent substrate; a lowrefractive index layer, whose refractive index is lower than that of thetransparent substrate, disposed over the transparent substrate; and asolgel film disposed over the low refractive index layer.
 2. The opticalsubstrate according to claim 1, wherein the refraction index of said lowrefractive index layer ranges from 1.003 to 1.400.
 3. The opticalsubstrate according to claim 1, wherein said low refractive index layeris silica aerogel layer.
 4. The optical substrate according to claim 1,wherein said solgel film has an Si—O—C structure.
 5. The opticalsubstrate according to claim 1, wherein said solgel film has in Itsinfrared absorption spectrum an absorption of 1107±2 cm⁻¹ in wave numberattributable to Si—O—C.
 6. The optical substrate according to claim 1,wherein said solgel film has in its infrared absorption spectrum anabsorption A of 1107±2 cm⁻¹ in wave number attributable to Si—O—C and anabsorption B of 1070±2 cm⁻¹ in wave number attributable to Si—O—Si, andthe intensity ratio NB between the absorption A and the absorption Branges from 0.5 to 1.0.
 7. The optical substrate according to claim 1,wherein a relationship between the surface roughness of said lowrefractive index layer represented by Ra1 and that of said solgel filmover said low refractive index layer represented by Ra2 is Ra1>Ra2. 8.The optical substrate according to claim 1, wherein the refraction indexof said solgel film is greater than that of said low refractive indexlayer.
 9. The optical substrate according to claim 1, wherein thethickness of said solgel film ranges from 0.05 to 1.0 μM.
 10. Theoptical substrate according to claim 1, wherein said solgel film isformed by using a precursor having an Si—H group.
 11. The opticalsubstrate according to claim 1, wherein said transparent substrate is aplastic substrate.
 12. The optical substrate according to claim 1,wherein a barrier layer is further disposed over said solgel film. 13.The optical substrate according to claim 1, wherein said transparentsubstrate is a substrate provided with a thin film transistor.
 14. Alight emitting element comprising: a transparent substrate; a lowrefractive index layer, whose refractive index is lower than that of thetransparent substrate, disposed over the transparent substrate; a solgelfilm disposed over the low refractive index layer, a first electrodeover the solgel film; a light emitting layer over the first electrode;and a second electrode over the light emitting layer.
 15. The lightemitting element according to claim 14, wherein the refraction index ofsaid low refractive index layer ranges from 1.003 to 1.400.
 16. Thelight emitting element according to claim 14, wherein said lowrefractive index layer is silica aerogel layer.
 17. The light emittingelement according to claim 14, wherein said solgel film has an Si—O—Cstructure.
 18. The light emitting element according to claim 14, whereinsaid solgel film has in its infrared absorption spectrum an absorptionof 1107±2 cm⁻¹ in wave number attributable to Si—O—C.
 19. The lightemitting element according to claim 14, wherein said solgel film has inits infrared absorption spectrum an absorption A of 1107±2 cm⁻¹ in wavenumber attributable to Si—O—C and an absorption B of 1070±2 cm⁻¹ in wavenumber attributable to Si—O—Si, and the intensity ratio A/B between theabsorption A and the absorption B ranges from 0.5 to 1.0.
 20. The lightemitting element according to claim 14, wherein a relationship betweenthe surface roughness of said low refractive index layer represented byRa1 and that of said solgel film over said low refractive index layerrepresented by Ra2 is Ra1>Ra2.
 21. The light emitting element accordingto claim 14, wherein the refractive index of said solgel film is greaterthan that of said low refractive index layer.
 22. The light emittingelement according to claim 14, wherein the thickness of said solgel filmranges from 0.05 to 1.0 μm.
 23. The light emitting element according toclaim 14, wherein said solgel film is formed by using a precursor havingan Si—H group.
 24. The light emitting element according to claim 14,wherein said transparent substrate is a plastic substrate.
 25. The lightemitting element according to claim 14, wherein a barrier layer isfurther disposed over said solgel film and said first electrode isdisposed over the barrier layer.
 26. The light emitting elementaccording to claim 14, wherein said transparent substrate is a substrateprovided with a thin film transistor.
 27. The light emitting elementaccording to claim 14, wherein said transparent substrate is providedwith a color conversion filter.
 28. The light emitting element accordingto claim 14, wherein said transparent substrate is provided with a colorfilter.
 29. The light emitting element according to claim 14, whereinlight emission from said light emitting layer is monochromatic.
 30. Thelight emitting element according to claim 29, wherein said monochromaticlight is white light.
 31. The light emitting-element according to claim29, wherein said monochromatic light is blue light.
 32. The lightemitting element according to claim 14, wherein said light emittingelement is an organic electroluminescence element.
 33. A display devicecomprising the light emitting element according to claim 14 and meansfor driving the light emitting element, which are disposed over thetransparent substrate.
 34. The display device according to claim 33,wherein said means for driving is a thin film transistor.
 35. Thedisplay device according to claim 34, further comprising a level gapeasing film for easing a convex attributable to said thin filmtransistor.
 36. The display device according to claim 33, wherein thefilm thickness of said low refractive index layer is 4 μm or less.
 37. Amethod of manufacturing an optical substrate comprising the steps ofdisposing a low refractive index layer over a transparent substrate;coating a solgel film over the low refractive index layer; andincreasing the density of the solgel film by irradiation withultraviolet rays after drying.
 38. A method of manufacturing a lightemitting element comprising the step of: disposing a low refractiveindex layer over a transparent substrate; coating a solgel film over thelow refractive index layer; increasing the density of the solgel film byirradiation with ultraviolet rays after drying; and forming a lightemitting element according to claim
 14. 39. A method of manufacturing adisplay device comprising the step of disposing a low refractive indexlayer over a transparent substrate provided with a driving means;coating a solgel film over the low refractive index layer; increasingthe density of the solgel film by irradiation with ultraviolet raysafter drying, and forming a light emitting element according to claim14.
 40. An optical substrate comprising: a transparent substrate; a lowrefractive index layer, whose refractive index is lower than that of thetransparent substrate, disposed over the transparent substrate; and asolgel film disposed over the low refractive index layer, wherein therefraction index of the low refractive index layer ranges from 1.003 to1.400; the low refractive index layer is silica aerogel layer; thesolgel film has an Si—O—C structure; a relationship between the surfaceroughness of the low refractive index layer represented by Ra1 and thatof the solgel film over the low refractive index layer represented byRa2 is Ra1>Ra2; and the refraction index of the solgel film is greaterthan that of the low refractive index layer.
 41. A light emittingelement comprising: a transparent substrate; a low refractive indexlayer, whose refractive index is lower than that of the transparentsubstrate, disposed over the transparent substrate; a solgel filmdisposed over the low refractive index layer; a first electrode over thesolgel film; a light emitting layer over the first electrode; and asecond electrode over the light emitting layer, wherein the refractionindex of the low refractive index layer ranges from 1.003 to 1.400; thelow refractive index layer is silica aerogel layer; the solgel film hasan Si—O—C structure; a relatonship between the surface roughness of thelow refractive index layer represented by Ra1 and that of the solgelfilm over the low refractive index layer represented by Ra2 is Ra1>Ra2;and the refractive index of the solgel film is greater than that of thelow refractive index layer.
 42. A display device comprising a lightemitting element, a thin film transistor for driving the light emittingelement and a level gap easing film for easing a convex attributable tothe thin film transistor, wherein the light emitting element comprises atransparent substrate; a low refractive index layer, whose refractiveindex is lower than that of the transparent substrate, disposed over thetransparent substrate; a solgel film disposed over the low refractiveindex layer, a first electrode over the solgel film; a light emittinglayer over the first electrode; and a second electrode over the lightemitting layer, wherein the refraction index of the low refractive indexlayer ranges from 1.003 to 1.400; the low refractive index layer issilica aerogel layer; the solgel film has an Si—O—C structure; arelationship between the surface roughness of the low refractive indexlayer represented by Ra1 and that of the solgel film over the lowrefractive index layer represented by Ra2 is Ra1>Ra2; the refractiveindex of the solgel film is greater than that of the low refractiveindex layer, and the film thickness of the low refractive index layer is4 μm or less.